OMC Cobra, 1986 3.0L 4cyl Having Carborator Starving Problems

[dale6998]

Hello,

I have a 1986 OMC Cobra 3.0L 4cyl Stern Drive.
I bought it last year, and rode around for about 2 months...maybe 20 hours of total drive time.
Then, I followed the Youtube video we've probably all seen in one form or another on how to winterize the boat and followed the instructions. I did, however, leave the gas (with marine fuel stabilizer) in the tank over the winter about 1/2 full.

A week ago, I pulled it out and had some troubleshooting to do with the oil and some cracked gaskets, so on the hose it went and during all this troubleshooting, the engine started fine and would idle for as long as I wished. No problem.

I then go to take it out for a ride today, put some 93 Octane fuel in it (11 gallons) and all of a sudden, after about 2 minutes of riding, it stalls...turn it over and no gas (just that awfully disappointing dry turning of the starter). I bring it home, hook it up to the hose and start with everything I can think of.

Two interesting things came out of my observations:
1) I had planned to suck the gas out and start fresh...So, I couldn't think of a good way to get to the tank other than the sending unit, so I took one of the screws out and it was like Old Faithful...A gas geyser! I thought...hmm, that's strange...better let that settle down, so I did.
2) I notice that if I loosen the top plate of the carburetor, the engine will start and run rough (obviously...the carburetor is half off!)....But, once the carburetor is screwed down tight, the engine fails and acts like it's getting no gas. Also, if you press it down so that the engine dies and then let it go, there's a noticeable swoooooosh sound (like a vacuum seal) that occurs when it's pulled up.


I've taken the fuel line off at the output of the fuel pump and turned the engine...I see plenty of gas pour out.
I've checked the fuel pump filter....Looks good to me.


I did put some of that Permatex Form-A-Gasket on the edge of the carburetor....

However, I guess my question is 'why would adding gas now make the carburetor so sensitive? do they really prefer running on old sour winter gas instead of fresh clean 93 octane?'

Follow up question is 'how do i fix this?'

Thanks!

 

[HT32BSX115]

Howdy,


Welcome aboard!


Well, you may have some other "things" going on there. I doubt the fuel is the culprit.

btw, I always store my boat with last years (Aug) fuel in the tank, 1/4 or so..... (I don't use fuel stabilizer) and fill it up next year(Jun) when I use it. Never had a fuel issue at all.

But it sounds like you may have a clogged fuel tank vent. Try running it until it quits and remove teh fuel tank cap and listen for a "sucking" sound. If there is one, your vent is likely plugged with critters & crud.

If that's not the problem, you should start at the tank and check everything.

1. Fuel tank for contamination,
2. fuel tank pickup screen,
3. Anti-siphon check valve,
4. REPLACE the filter
5. fuel line integrity (if it leaks prior to the pump, it will suck air....... If it leaks after the fuel pump, it'll leak fuel)
6. carburetor. It may be contaminated with crud/water/etc. get a rebuild kit. Don't use Permatex when you reassemble.

Regards,


Rick

 

[dale6998]

Hi Rick,

Thanks for the response...I'm not sure where the items you list in step 2 and 3 are.
Could you eleaborate on where those are located/how to get to them? I have the Solec manual for the motor, but it doens't mention anything about the exterior fuel system.

Also, as a further diagnostic bit of data (since I didn't see this in time), I drained the fuel with a hand pump/vacuum making plastic tank. The fuel flew out with no problems all the way down to the end...Would this imply that the item #2 is good?

Also, this problem persists even if the fuel cap is off...Would that rule out item #3? And it's a sucking sound right in the carburetor after stalling that occurs when lifting the top-plate off the float bowl.

Thanks!
--Dale

 

[HT32BSX115]

Hi Rick,

Thanks for the response...I'm not sure where the items you list in step 2 and 3 are.
Could you eleaborate on where those are located/how to get to them? I have the Solec manual for the motor, but it doens't mention anything about the exterior fuel system.

Also, as a further diagnostic bit of data (since I didn't see this in time), I drained the fuel with a hand pump/vacuum making plastic tank. The fuel flew out with no problems all the way down to the end...Would this imply that the item #2 is good?

Also, this problem persists even if the fuel cap is off...Would that rule out item #3? And it's a sucking sound right in the carburetor after stalling that occurs when lifting the top-plate off the float bowl.

Thanks!
--Dale

Dale,

The anti-siphon check valve is usually right at the tank (or screwed into the tank and the fuel line connects to it. The pickup screen is on the tube that goes to the bottom of the tank (inside) .

If you used a pump to "suck" the fuel out (presumably using the existing fuel line?) and you were able to get it to flow out, it would seem that the fuel flow from the tank is adequate. (and the anti-siphon valve and screen are NOT restricting flow) but they may still be doing it enough to prevent your engine driven fuel pump from working though , OR the fuel pump is defective and any little bit of restriction is not over come by it.

Those fuel pumps are pretty simple, diaphragm, 2 check valves. If the check valves are being held open by debris or corrosion, the pump will not pump! If there's a hole or rip in the diaphragm, it won't work either. If the pump is original, (~30 years old) It's probably time to replace it anyway.

A good way to check pump operation would be to get a fuel gage (0-15 PSI) and check fuel pressure when everything is hooked up. 5-7 psi or so is good.....

You need to determine where the problem is to fix it.

It's either blockage at the fuel tank (AS-valve, pick-up screen) , fuel filter (carburetor fuel filter if equipped), defective pump or plugged carb etc.


Regards,


Rick

 

[JJBoatr]

Check the carb inlet filter. It is where the fuel line connects to the carb either on the front or on the side. It should be there whether you have a Holley or Rochester. The fact that the engine stumbles when you start disassembling the carb is irrelevant. You are creating a vacuum leak and the carb cannot compensate for the extra air that the engine is sucking in. You are creating a lean condition when you do this, I would stop if I were you.